Automated scanning systems for non-destructive inspection of curved cylinder-like workpieces

ABSTRACT

Systems and methods for high-speed non-destructive inspection of a half- or full-barrel-shaped workpiece, such as a barrel-shaped section of an aircraft fuselage. Such workpieces can be scanned externally using a mobile (e.g., translating) arch gantry system comprising a translatable arch frame disposed outside the fuselage section, a carriage that can travel along a curved track carried by the arch frame, a radially inward-extending telescopic arm having a proximal end fixedly coupled to the carriage, and an NDI sensor unit coupled to a distal end of the telescoping arm. The stiffeners of the fuselage sections can be scanned using a mobile scanner platform disposed inside the fuselage section, which platform comprises a radially outward-extending telescopic arm rotatably coupled to a mobile (e.g., holonomic or linear motion) platform and an NDI sensor unit coupled to a distal end of the telescoping arm. The scan data is matched with position data acquired using any one of a plurality of tracking systems to enable the display of NDI features/flaws on a three-dimensional representation of the workpiece.

RELATED PATENT APPLICATIONS

This application is a divisional of and claims priority from U.S. patentapplication Ser. No. 15/809,376 filed on Nov. 10, 2017, which in turn isa divisional of and claims priority from U.S. patent application Ser.No. 14/279,355 filed on May 16, 2014 (issued on Dec. 5, 2017 as U.S.Pat. No. 9,834,323).

BACKGROUND

This disclosure generally relates to non-destructive inspectionequipment and methods, and relates more particularly to methods andapparatus for inspecting barrel-shaped structures made of compositematerial.

Non-destructive inspection (NDI) of structures involves thoroughlyexamining a structure without harming the structure or requiringsignificant disassembly. Inspection may be performed duringmanufacturing of a structure and/or after a structure has been put inservice to determine the condition, quality, or structural state of thestructure.

The production manufacturing of a large composite structure for anactive airplane program needs to be done at a rate that meets schedulecommitments. Non-destructive inspection of primary structure is anecessary part of the manufacturing process and must be done at a ratecapable of keeping up with the published schedule.

For example, it is known to fabricate barrel-shaped fuselage sectionsmade of composite material on an assembly line with high through-put.The finished fuselage sections need to undergo NDI at a high rate. Someexisting solutions for inspecting barrel-shaped fuselage sections arelarge, expensive multiple-axis robotic systems which move ultrasonictransducer arrays over the outer mold line (OML) of the fuselage sectionusing encoded rails and end effectors guided to follow pre-programmedpaths.

In addition, high-speed non-destructive inspection of stiffeners(stringers) on the inside of many composite airplane fuselage sectionsis desirable in order to maintain the manufacturing rate. One existingsolution involves rolling the barrel-shaped fuselage section in arotating tool frame that allows each stiffener being inspected to beunder a robotic crawler, so gravity is not an issue, and the stiffenercrawlers holding multiple ultrasonic transducer (UT) arrays can crawl ona relatively horizontal surface.

There is a need for improvements in systems and methods fornon-destructive inspection of barrel-shaped workpieces (such as fuselagesections that may include internal stiffening elements) that facilitatea high rate of production.

SUMMARY

The subject matter disclosed herein is directed to systems and methodsfor high-speed non-destructive inspection of a curved cylinder-likeworkpiece (e.g., in the shape of a half or full barrel). The curvedcylinder-like workpiece may be a large-scale part, thereby takingadvantage of automation provided by embodiments disclosed herein. Givenby way of non-limiting example for illustration purposes only, thecurved cylinder-like workpiece may be an aircraft part, such as abarrel-shaped section of an aircraft fuselage. It should be appreciated,however, that the systems and methods described hereinafter withreference to a fuselage section may also be applied to other types ofcurved cylinder-like workpieces. Furthermore, the curved cylinder-likeworkpiece need not be related to an aircraft, and can be a part of someother type of vehicle or structure.

Moreover, the curved cylinder-like workpiece may be made of any materialas desired for a particular application. It will be appreciated that thetype of material used for the curved cylinder-like workpiece may, inpart, determine which type of non-destructive inspection technique isused to inspect the curved cylinder-like workpiece. Given by way ofnon-limiting examples, the curved cylinder-like workpiece may be made ofcomposite material, such as a composite laminate made offiber-reinforced plastic, or a metal, such as aluminum or titanium. Itwill be understood that it is not intended to limit in any mannerwhatsoever the materials from which the curved cylinder-like workpiecemay be made.

Depending on the type of material being inspected, any one of amultiplicity of types of NDI sensors can be utilized. A variety of typesof NDI sensors suitable for use with the scanning apparatus disclosedherein are listed and described in U.S. Pat. No. 7,743,660 entitled“System and Method for Automated Inspection of Large-Scale Part”.

In accordance with some embodiments disclosed herein, the NDI sensorunits are supported by apparatus that travels along tracks. As usedherein, the term “tracks” encompasses rails, grooves, guide surfaces,and equivalents thereof. A track may be straight (i.e., linear) orcurved. For example, for externally scanning a workpiece that extendslongitudinally and circumferentially, an NDI sensor unit may be mountedto a carriage that travels circumferentially along a curved track formedby guide surfaces, which curved track in turn is mounted to an archframe that travels longitudinally along linear tracks in the form ofrails.

For the sake of illustration, systems and methods for high-speedultrasonic inspection of stiffened barrel-shaped (e.g., half or fullbarrel) fuselage sections made of composite material will be disclosedin detail. However, it should be appreciated that the apparatusdisclosed herein can be employed in the non-destructive inspection ofbarrel-shaped workpieces other than fuselage section using NDI sensorunits other than UT arrays.

In the context of the specific application of inspecting fuselagesections, the scanning system may comprise means for scanning the skinof the fuselage section from a vantage point external to the fuselagesection and means for scanning substructure, such as stringers attachedto the inside of a stiffened fuselage section. The means for scanningthe stiffeners on the inside of a fuselage section can work in concertand concurrently with the means that scan the fuselage sectionexternally. In the alternative, the external and internal scanning canbe performed at different times and/or at different places. The fuselagesections can be scanned externally before or after the stiffeners havebeen attached. In the embodiments disclosed below, the scanning meanscomprise multiple linear UT arrays that collect wide swaths ofultrasonic data. In one configuration, some UT arrays sweep the outermold line of the fuselage section circumferentially, while other UTarrays travel longitudinally along the length of the stiffeners attachedto the inside of the fuselage section.

In accordance with some embodiments, the fuselage sections (or otherworkpieces) can be scanned externally using a mobile (e.g., translating)arch gantry system comprising a translatable arch frame disposed outsidethe fuselage section, a carriage that can travel along a curved trackcarried by the arch frame, a radially extending telescopic arm having aproximal end fixedly coupled to the carriage, and an NDI sensor unitcoupled to a distal end of the telescoping arm. The stiffeners of thefuselage sections can be scanned using a mobile scanner platformdisposed inside the fuselage section, which platform comprises aradially extending telescopic arm rotatably coupled to a mobile (e.g.,holonomic or linear motion) platform and an NDI sensor unit coupled to adistal end of the telescoping arm. The scan data is matched withposition data acquired using any one of a plurality of tracking systemsto enable the display of NDI features/flaws on a three-dimensionalrepresentation of the workpiece.

For fuselage sections having a half barrel shape, the entire OML of thehalf-barrel fuselage section can be scanned externally using an archgantry system by moving the latter in increments from one end of thefuselage section to the other end, stopping after each incrementaladvance to perform a circumferential scan of a respective swath of thefuselage section. In one embodiment for scanning of fuselage sectionshaving a full barrel shape, one half of the full-barrel fuselage sectioncan be scanned externally from one end to the other; then the fuselagesection is rotated 180 degrees about its longitudinal axis in the archgantry system and then the other half of the fuselage section can bescanned externally from one end to the other.

One aspect of the subject matter disclosed in detail below is a methodfor scanning a workpiece having a curved section that extendslongitudinally and circumferentially, the method comprising: (a) movinga curved track to a first longitudinal position relative to theworkpiece, the curved track being disposed radially outward from thecurved section of the workpiece; (b) moving an NDI sensor unit along thecurved track while the curved track is stationary at the firstlongitudinal position; (c) during step (b), activating the NDI sensorunit to inspect a first strip-shaped area of the curved section of theworkpiece; (d) processing signals output from the NDI sensor unit toderive a first strip of scan data characterizing a structural state ofthe first strip-shaped area of the curved section of the workpiece; (e)during step (b), acquiring location data representing locations of theNDI sensor unit relative to the workpiece; and (f) mapping the firststrip of scan data to a three-dimensional model of the workpiece basedon the location data acquired in step (e). The workpiece may be afuselage section made of composite material. The method may furthercomprise displaying features overlaid on a representation of a portionof the three-dimensional model of the workpiece based on the results ofsteps (d) and (f). The method may further comprise the following steps:(g) subsequent to step (b), moving the curved track from the firstlongitudinal position to a second longitudinal position relative to theworkpiece; (h) moving the NDI sensor unit along the curved track whilethe curved track is stationary at the second longitudinal position; (i)during step (h), activating the NDI sensor unit to inspect a secondstrip-shaped area of the curved section of the workpiece; (j) processingsignals output from the NDI sensor unit to derive a second strip of scandata characterizing a structural state of the second strip-shaped areaof the curved section of the workpiece; (k) during step (h), acquiringlocation data representing locations of the NDI sensor unit relative tothe workpiece; and (l) mapping the second strip of scan data to thethree-dimensional model of the workpiece based on the location dataacquired in step (k).

Another aspect is a system for external scanning of a workpiece having acurved outer mold line, the system comprising: first and second lineartracks which are mutually parallel; a curved track disposed in a planegenerally transverse to the first and second linear tracks, the curvedtrack being coupled to and translatable along the first and secondlinear tracks; a carriage coupled to and movable along the curved track;an extendible arm having a proximal end coupled to the carriage; an NDIsensor unit coupled to a distal end of the extendible arm; a locationtracking system capable of tracking the location of the NDI sensor unitrelative to the workpiece; a data processing system capable of receivingscan data from the NDI sensor unit and location tracking data from thelocation tracking system and then correlating the scan data with thelocation tracking data; and a display system capable of displaying thescan data on a three-dimensional representation of the workpiece (suchas with texture maps) based on results of the correlating processperformed by the data processing system.

A further aspect is a system for scanning a workpiece having a curvedsection that extends longitudinally and circumferentially, the systemcomprising: a pair of linear tracks parallel to a longitudinaldirection; an arch frame that extends circumferentially and is arrangedto travel along the linear tracks; a first actuator which, whenactivated, causes the arch frame to travel along the linear tracks; acurved track supported by the arch frame; a carriage arranged to travelalong the curved track; a second actuator which, when activated, causesthe carriage to travel along the curved track; an extendible armcomprising a first member mounted to the carriage and a second memberwhich is arranged to translate relative to the first member; a thirdactuator which, when activated, causes the second member to translaterelative to the first member; and an NDI sensor unit mounted to thesecond member and operable to acquire scan data during its motion.

Yet another aspect of the disclosed subject matter is a system forscanning a substructure of a curved cylinder-like workpiece, whichsubstructure extends along an inner surface of the workpiece. The systemcomprises: a mobile platform comprising a frame; a first actuator which,when activated, exerts a force urging the mobile platform to move; anextendible arm comprising a first member pivotably mounted to the frameof the mobile platform and a second member which is translatablerelative to the first member; a second actuator which, when activated,exerts a force urging the second member to translate relative to thefirst member; and a first NDI sensor unit coupled to a distal end of theextendible arm; an encoder device capable of outputting signalsrepresenting incremental movements of the first NDI sensor unit along asubstructure; and a computer system programmed to perform the followingoperations: controlling the first and second actuators and the first NDIsensor unit; receiving signals from the encoder device; convertingsignals from the encoder device into position data representing aposition of the first NDI sensor unit along the substructure; receivingscan data from the first NDI sensor unit; and correlating the scan datawith the position of the first NDI sensor unit along the substructure.The mobile platform may further comprise a plurality of omnidirectionalwheels rotatably coupled to the frame or a plurality of rollersrotatably coupled to the frame which roll on linear tracks.

A further aspect is a system for scanning a stiffened curvedcylinder-like workpiece, the system comprising: first means forcircumferentially scanning an outer mold line of the workpiece; andsecond means for scanning a longitudinal stiffener attached to an innermold line of the workpiece, wherein the first and second means work inconcert and concurrently. The system may further comprise an arch framedisposed outside the workpiece and comprising a curved track, the archframe being translatable along a longitudinal axis of the workpiece,wherein the first means comprise: a carriage that can travel along thecurved track; a radially extending telescopic arm having a proximal endfixedly coupled to the carriage; and an NDI sensor unit coupled to adistal end of the telescoping arm. In addition, the system may furthercomprise a mobile scanner platform disposed inside the workpiece,wherein the second means comprise: a radially extending telescopic armrotatably coupled to the mobile scanner platform; and an NDI sensor unitcoupled to a distal end of the telescoping arm.

Other aspects of systems and methods for NDI scanning of a curvedcylinder-like workpiece with and without longitudinal stiffeners aredisclosed below.

BRIEF DESCRIPTION OF THE DRAWINGS

Drawings representing views of physical structures are not drawn toscale. Also, it should be noted that the drawings representing physicalstructures do not show the flexible cables which electrically connectthe hardware to a computer system (not shown) and, in the case ofultrasonic inspection, which connect an ultrasonic transducer inspectionunit to a source of acoustic couplant (e.g., water).

FIG. 1 is a diagram representing an isometric view of portions of anexternal scanning system for non-destructive inspection of the OML of afuselage section in accordance with one embodiment.

FIG. 2 is a diagram representing an end view of portions of an externalscanning system having an NDI sensor unit whose location is tracked by amotion capture system.

FIG. 3 is a diagram representing a direct three-dimensional (3-D)overlay of NDI data onto a 3-D model of a half-barrel fuselage section.

FIG. 4 is a diagram representing an elevational view of an NDI scanningassembly designed to sweep circumferentially along an arch frame of amobile arch gantry system in accordance with an alternative embodiment.

FIG. 5 is a diagram showing a schematic view of a motion capture systemfor tracking the location of an NDI sensor unit relative the frame ofreference of an object (e.g., a workpiece) being scanned in accordancewith one embodiment.

FIG. 6 is a block diagram showing components of a control system inaccordance with another embodiment that uses encoders to track therelative location (e.g., relative to an initial location acquired usinga local positioning system) of an NDI sensor unit mounted to an externalscanning system of the type partly depicted in FIG. 1.

FIG. 7A is a diagram representing an end view of portions of an internalscanning system for scanning stringers of a half-barrel fuselagesection, such system having a single arm configuration and comprising aholonomic motion platform with motion capture tracking.

FIG. 7B is a diagram representing an end view of portions of an internalscanning system for scanning stringers of a half-barrel fuselagesection, such system having a single arm configuration and comprising aholonomic motion platform with LPS (Local Positioning System) andencoder tracking.

FIG. 8 is a diagram representing an end view of portions of an internalscanning system for scanning stringers of a half-barrel fuselagesection, such system having a double arm configuration and comprising aholonomic motion platform with LPS and encoder tracking.

FIG. 9 is a diagram representing an end view of portions of an internalscanning system for scanning stringers of a half-barrel fuselagesection, such system having a double arm configuration and comprising alinear motion platform on linear bearings with LPS and encoder tracking.

FIG. 10 is a diagram representing an end view of portions of an internalscanning system for scanning stringers of a half-barrel fuselagesection, such system having a single arm, double end-effectorconfiguration in accordance with one embodiment and comprising a linearmotion platform on linear bearings with LPS and encoder tracking.

FIG. 11 is a diagram representing an end view of portions of an internalscanning system for scanning stringers of a half-barrel fuselagesection, such system having a single arm, double end-effectorconfiguration in accordance with another embodiment and comprising alinear motion platform on linear bearings with LPS and encoder tracking.

FIGS. 12 and 13 are diagrams representing respective end views ofportions of an internal scanning system for scanning stringers of afull-barrel fuselage section, such system having a single arm, doubleend-effector configuration of the type shown in FIG. 10 and comprising alinear motion platform on rails. The linear motion platform is shown inrespective locations for scanning stringers on the upper half (FIG. 12)and lower half (FIG. 13) of a full-barrel fuselage section.

FIG. 14 is a diagram showing a top view of some components of the systemdepicted in FIGS. 12 and 13. The dashed rectangle indicates thelongitudinal and transverse dimensions of a full-barrel fuselage sectionsurrounding a linear motion platform on rails.

FIG. 15 is a diagram showing a physical setup in which a localpositioning system is used to provide a location of a linear motionplatform relative to a half-barrel fuselage section.

FIG. 16 is a flow diagram of an aircraft production and servicemethodology.

FIG. 17 is a block diagram showing systems of an aircraft.

Reference will hereinafter be made to the drawings in which similarelements in different drawings bear the same reference numerals.

DETAILED DESCRIPTION

Embodiments of apparatus and methods for non-destructive inspection ofcurved cylinder-like workpieces in the form of half- orfull-barrel-shaped sections of an aircraft fuselage will now bedescribed in detail for the purpose of illustration. The apparatus andmethods disclosed herein may also be used for similar applications whichrequire non-destructive inspection, including other curved cylinder-likeworkpieces. For example, such workpieces may comprise shells orhalf-shells having a curved cross-sectional profile (e.g., an oval,ellipse, or circle, or any section thereof) that is constant or variessmoothly in a longitudinal direction.

FIG. 1 represents an isometric view of portions of an external scanningsystem for ultrasonic inspection of the OML of a half-barrel fuselagesection 12 in accordance with one embodiment. (The same externalscanning system can be adapted to inspect respective halves of the OMLof a full-barrel fuselage section.) Means for supporting the half-barrelfuselage section 12 are not shown in FIG. 1.

During ultrasonic inspection, an NDI sensor unit 8 is scannedcircumferentially (i.e., along a Y-axis) across the OML of thehalf-barrel fuselage section 12 at successive longitudinal positions(i.e., spaced along an X-axis). In the embodiments disclosed herein, theNDI sensor unit 8 takes the form of multiple ultrasonic transducer (UT)elements lined up in an array and contained in a structure called ashoe, mounted to an end effector (not shown), to provide for a widescan. During each circumferential scan, the NDI sensor unit 8 acquires arespective swath of ultrasound scan data. Successive swaths ofultrasound scan data may be acquired from successive contiguous segmentsof the half-barrel fuselage section 12 to provide full scan coveragefrom one end of the half-barrel fuselage section 12 to the other end.

In accordance with the embodiment depicted in FIG. 1, the NDI sensorunit 8 is supported by a translatable arch gantry system 30 comprising apair of arch frame members 32 a, 32 b connected at their opposing endsto gantry trolleys 34 a and 34 b. The gantry trolleys 34 a and 34 b rideon linear rails (not shown in the drawings) of respective railassemblies 28 a and 28 b. In one implementation, the gantry trolleys 34a and 34 b may comprise rollers that roll along the linear rails.Alternatively, other systems for guiding linear motion could beemployed. For example, the gantry trolleys 34 a and 34 b may be equippedwith sliders comprising respective pairs of recirculating ball bearingsthat roll along a pair of linear guide tracks. Optionally, the Xposition of each gantry trolley 34 a, 34 b can be measured by arespective position sensor (e.g., an encoder) to provide feedback to atrolley motion control subsystem (not shown in FIG. 1) which controlsthe X-direction motion of the arch gantry system 30. The half-barrelfuselage section 12 is positioned so that its longitudinal axis (notshown in the drawings) will be parallel to the X axis.

The arch frame members 32 a and 32 b are provided with mutually parallelcurved tracks (not visible in FIG. 1) preferably disposed in planestransverse to the linear rails of rail assemblies 28 a and 28 b. The NDIsensor unit 8 may be coupled to a carriage 52 which is designed totravel along the curved tracks. As the carriage 52 travels along thecurved tracks (while the arched gantry system 30 is stationary), the NDIsensor unit 8 can be activated to scan a circumferential portion of theOML of the half-barrel fuselage section 12 from one side edge to theother side edge. The curved tracks may be constructed so that theircurvatures approximate the curvature of the workpiece, with a compliantend effector compensating for slight to moderate curvature mismatch. Thecurvature of the half-barrel fuselage section 12 may be circular,elliptical, oval, or some other shape.

In accordance with one embodiment, the carriage 52 may compriserespective motorized trolleys which travel along the curved tracks ofthe arch frame members 32 a and 32 b, thereby scanning the NDI sensorunit 8 circumferentially across the OML of the half-barrel fuselagesection 12. In some embodiments, after each complete pass over the fullhalf-circumference of the half-barrel fuselage section 12, theextendible arm 62 is retracted to lift the NDI sensor unit 8 from thesurface and returned to its starting position along the arch framemembers 32 a and 32 b. In other embodiments, the extendable arm 62 isshifted over to the location of the next strip without retracting andreturning to its starting position. This allows the NDI scanning processto run in the opposite direction (saving time by scanning on the returntrip). At the same time, the arch gantry system 30 is automaticallymoved along the X-axis by a distance equal to the width of the swath ofscan data acquired. This process is repeated until the entire OML of thehalf-barrel fuselage section 12 has been scanned. Then the half-barrelfuselage section 12 is removed and the next half-barrel fuselage sectionis placed in position for external scanning (or in some embodiments, theinterior is scanned by another system before the barrel is moved out).In the case of external scanning of a full-barrel fuselage section,first one half of the full barrel is scanned; then the full barrel isrotated 180 degrees and the other half is scanned. Then the full barrelis removed and a next full-barrel fuselage section is placed in positionfor external scanning.

In the alternative, two or more arch gantry systems of the type shown inFIG. 1 can be mounted to the rail assemblies 28 a and 28 b for scanningrespective portions of the half-barrel fuselage section 12 concurrently.By operating multiple NDI sensor units concurrently, the rate ofinspection can be increased.

FIG. 2 is an end view of portions of an external scanning system of thetype shown in FIG. 1 wherein the location of the NDI sensor unit 8 istracked using a motion capture system. (As in FIG. 1, the means forsupporting the half-barrel fuselage section 12 are not shown in FIG. 2.)The NDI sensor unit 8 can be coupled to the carriage (not visible inFIG. 2) by means of an extendible arm 62. The extendible arm 62 may bein a retracted position during longitudinal movement of the arch gantrysystem 30. Prior to initiation of a circumferential scanning operation,the extendible arm 62 can be actuated to extend, bringing the shoe ofthe NDI sensor unit 8 into contact with the OML of the fuselage section12. (In alternative applications of the scanning system in which the NDIsensor unit does not include contact-type sensors, the extendible armmay be extended to bring the NDI sensor unit into proximity withoutcontacting the OML of the workpiece.)

The motion capture hardware partly depicted in FIG. 2 may be of the typeused in the process disclosed in U.S. patent application Ser. No.13/744,730 entitled “Motion Capture Tracking for NondestructiveInspection”. As seen in FIG. 2, the motion capture system comprises amultiplicity of motion capture cameras 2, a plurality ofretro-reflective markers 36 (a.k.a. retroreflectors) attached(temporarily) to the half-barrel fuselage section 12, and a plurality ofretro-reflective markers 10 attached to the shoe of the NDI sensor unit8. Other components of the motion capture system will be described laterwith reference to FIG. 5. Initially, the motion capture system is turnedon and correlated to the optical targets on the shoe of the NDI sensorunit 8. Then the motion capture system can be used to correlate thelocation of the NDI sensor unit 8 with the collected ultrasound scandata.

During external ultrasonic inspection of a fuselage section, typical NDIscanner software is capable of processing X and Y position data in theform of respective sets of quadrature pulses (X and Y). As will beexplained in detail below with reference to FIG. 5, the motion capturesystem uses a simulated X and Y encoder pulse method (using a separatedata acquisition device to generate the pulses) as described in U.S.patent application Ser. No. 13/744,730 (cited above) and in U.S. patentapplication Ser. No. 13/470,125 entitled “Automated Inspection of SparWeb in Hollow Monolithic Structure”. Overall, this would give betterabsolute positioning data than using encoder wheels, since encoderwheels on occasion are susceptible to slippage.

Full 3-D precision positioning information can be provided by areal-time motion capture system of the type disclosed in U.S. patentapplication Ser. No. 13/744,730. The scan data can then be overlaid ontoa 3-D model of the half-barrel fuselage section 12, so the scan data canbe tied to geometry and particularly to stringer locations and features.

FIG. 3 is a diagram representing a direct 3-D overlay of strips 42 ofscan data onto a 3-D model 40 of a half-barrel fuselage section 12. Inone implementation, the NDI sensor unit 8 acquires a respective strip 42of scan data (comprising returned ultrasound echo signal amplitude andtime-of-flight) during each circumferential scan. At the same time, themotion capture system acquires motion/position data which is already in3-D space that maps directly onto the 3-D model 40 of a half-barrelfuselage section 12. This allows the respective scan strips 42 to bemapped precisely onto the 3-D model 40 of a half-barrel fuselage section12. The overlay of scan data with the 3-D model 40 enables improved dataanalysis and potential automated data analysis as well. For example,features/flaw indications 44 and 45 can be directly correlated to thefuselage structure by direct overlay of scan data on the 3-D model. Inaddition, the direct data overlay onto the model can be used todetermine the thickness of a local part 46, which is needed for porosityquantification. In one embodiment, the process involves application ofNDI scan data strips as one or more computer graphics texture maps,which are projected onto the 3-D model surfaces in the virtualenvironment.

FIG. 4 is a diagram representing an elevational view of an NDI scanningassembly designed to sweep circumferentially along an arch frame 50 of amobile arch gantry system in accordance with an alternative embodiment.The arch frame 50 may be constructed to provide guide surfaces forguiding the motion path of a carriage 52 that carries a NDI sensor unit8. In the implementation depicted in FIG. 4, the carriage 52 may beprovided with a first set of rollers 54 (only two of which are visiblein FIG. 4) which roll along respective inner circumferential guidesurfaces of the arch frame 50 and a second set of rollers 56 (only twoof which are visible in FIG. 4) which roll along respective verticalguide surfaces of the arch frame 50. Optionally, a third set of rollersmay be provided which roll along respective outer circumferential guidesurfaces of the arch frame 50. The carriage 52 will follow a generallycircumferential path as it travels along the curved tracks formed by theguide surfaces. In the alternative, arched frame could be constructedwith curved rails and the carriage 52 could be provided with diametralsets of rollers which roll along the respective curved rails (in amanner similar to the rollers 132 which roll along linear rails 130 a,130 b depicted in FIG. 12, to be described in detail later).

The NDI scanning assembly shown in FIG. 4 further comprisescircumferential motion actuator in the form of a motorized rack andpinion subsystem. This rack and pinion subsystem comprises a curved rack58 attached to the arch frame 50 and a pinion gear 60 having teeth whichinterengage the teeth of curved rack 58. The guide surfaces that guiderollers 54 and the curved rack 58 may be semi-circular and mutuallyconcentric. However, the curvature of the guide surfaces that guiderollers 54 and the curved rack 58 may vary from semi-circular dependingon the shape of the OML of the fuselage section being inspected.

In accordance with the implementation shown in FIG. 4, the pinion gear60 is mounted to an end of an output shaft of a stepper motor 61 that ismounted to the carriage 52. Rotation of pinion gear 60 causes thecarriage 52 to travel along the curved tracks, during which the NDIsensor unit will be scanning the OML of the fuselage sectioncircumferentially. In the alternative, other known circumferentialmotion actuators could be employed, such as a cable system capable ofpulling the carriage in one direction or in an opposite direction.

Still referring to FIG. 4, the NDI scanning assembly may furthercomprise an extendible arm 62 in the form of a telescoping armcomprising an outer sleeve 64 attached to the carriage 52 and an innersleeve 66 which is axially translatable inside the outer sleeve 64.Linear motion of the inner sleeve 66 can be actuated via hydraulics,pneumatics, or a motor turning a threaded screw. In the embodiment shownin FIG. 4, the linear motion actuator comprises a lead screw (not shown)coupled to the output shaft of a motor 68 (for example, a stepper motor)and threadably coupled to a nut (not shown) attached to the inner sleeve66. In response to activation of motor 68, the inner sleeve 66 and theNDI sensor unit 8 coupled to inner sleeve 66 can be extended orretracted, to allow the operator to set the nominal length or retractthe arm during insertion of the fuselage section.

In accordance with a further feature, the NDI sensor unit 8 can bespring loaded to account for variation in the external surface of thefuselage section. More specifically, the NDI sensor unit 8 can becoupled to the inner sleeve 66 by means of a compliant support structure70 that both urges the shoe of the NDI sensor unit 8 toward the OML ofthe fuselage section and flexes to allow the NDI sensor unit 8 to adjustits radial position to take into account variations in the OML of thefuselage section and minor misalignments. For example, the compliantsupport structure 70 may take the form of flexible couplings. Inaccordance with one implementation, each flexible coupling may take theform of an aluminum rod having a spiral slot cut through the length ofthe aluminum tube to form a helical coil in a center section that actsas a spring. The flexure allowed by the center portion of the couplingaccommodates angular, parallel and axial misalignment between theextendible arm 62 and the shoe of the NDI sensor unit 8. Such flexiblecouplings are commercially available from Lovejoy, Inc., Downers Grove,Ill. Further details concerning use of such flexible couplings toprovide compliant motion of an ultrasonic transducer array relative to avariable scanned surface can be found in U.S. patent application Ser.No. 13/975,599 entitled “Apparatus for Non-Destructive Inspection ofStringers”.

FIG. 5 shows a basic system configuration of a motion capture system fortracking the location of an NDI sensor unit 8 relative to the frame ofreference of a fuselage section 12 (only partially shown in FIG. 5).Multiple motion capture cameras 2 (at least two) are set up around thefuselage section 12 to be scanned to create a three-dimensional capturevolume V that captures motion for all six degrees-of-freedom (6-DOF) ofthe NDI sensor unit 8 being tracked (3-DOF position: x (same as X seenin FIG. 1), y (a first component of Y seen in FIG. 1), z (a secondcomponent of Y seen in FIG. 1); and 3-DOF orientation: roll, pitch,yaw). Multiple objects in the capture volume can be trackedsimultaneously, e.g., in an inspection scenario where multiple NDIsensor units are scanning the fuselage section 12. Each NDI sensor unit8 to be tracked has a respective group (at least three) of passiveretro-reflective markers 10 attached thereto, the markers of each groupbeing arranged in a respective unique, non-collinear pattern. In theexample shown in FIG. 5, the NDI sensor unit 8 has four retro-reflectivemarkers 10. In the case of the embodiment partially depicted in FIG. 2,the fuselage section also has a group of retro-reflector markers 36,which are used by the motion capture system during an alignmentprocedure that enables a determination of the location of the NDI sensorunit 8 in the frame of reference of the fuselage section 12. In oneembodiment, a known marker pattern is placed in a known location on thetarget object and the alignment offset (position and orientation)between the target object coordinate system and the motion capturecoordinate system is determined. The markers of each group are arrangedin known patterns, and the information for defining the patterns isstored in the motion capture processor 14. A marker pattern can bedefined relative to a specific location on the NDI sensor unit 8 so thatthe marker pattern origin aligns with the origin of the NDI sensor unit;or in the alternative, the marker pattern can be attached to the NDIsensor unit 8 and then the offset position and orientation between theorigin of the marker pattern and the origin of the NDI sensor unit 8 isdetermined and used in a matrix transformation multiplication. Theresult from either approach is that the position and orientation of themarker pattern is defined relative to the origin of NDI sensor unit 8.

Each motion capture camera 2 seen in FIG. 5 can be a video camera of thetype comprising a ring of LEDs 6 surrounding a camera lens 4. Inconjunction with such cameras, each retro-reflective marker 10 maycomprise a hemispherical or ball-shaped body coated withretro-reflective paint, that reflects impinging light from the LEDs 6 ofeach camera 2 back toward the associated lens 4 of the respective camerain a well-known manner (i.e., the reflected light beam is substantiallyparallel to the transmitted light beam). In one known implementation,the retro-reflective marker comprises a surface coating made of a largenumber of micro-spheres, each of which are a refracting optical elementin the form of a transparent sphere and a reflective surface in the formof a hemi-spherical mirror. The motion capture system utilizes datacaptured from image sensors inside the cameras 2 to triangulate thethree-dimensional position of the NDI sensor unit 8 between multiplecameras configured to provide overlapping projections.

The outputs from cameras 2 are input to respective ports of motioncapture processor 14. The motion capture processor 14 collects real-timeimage information from all of the motion capture cameras 2, processesthe image data, and sends the information along a dedicated connectionto a motion tracking and applications computer 16, which has a displaymonitor 18 associated therewith for displaying the processed image data.Alternatively, the software functions executed by motion captureprocessor 14 and motion tracking and applications computer 16 can beexecuted by a single computer, i.e., the two hardware components can beintegrated inside one enclosure.

At each frame update, the positions of all of the retro-reflectivemarkers 10 in the capture volume V can be captured by each camera 2 andconverted by the motion capture processor 14 into three-dimensionalcoordinates, which are then associated with the known marker patternattached to the NDI sensor unit 8, resulting in full 6-DOF position andorientation representations for the NDI sensor unit 8. A separate dataconversion application running on motion tracking and applicationscomputer 16 accesses this object position/orientation data (alsoreferred to herein as “location data”) through a network socketconnection to the motion capture processor 16.

The data conversion application transforms the location data into thecoordinate system for the NDI scan, and then converts the coordinatedata into X and Y quadrature encoder pulse instructions, which adopt thesame format as pulses from a commercially available position encoder.The data acquisition device 20 converts the X and Y quadrature pulseinstructions into electrical signals that simulate X and Y encoderpulses. These simulated X and Y encoder pulses are sent by the dataacquisition device 20 to the NDI scanner hardware 22. In accordance withone embodiment, the data acquisition device 20 is a hardware componentthat takes pulse instructions (e.g., 1's and 0's from the motiontracking and application computer 16) and converts them into actualelectrical pulses at the correct voltage, which pulses are sent oversignal wires to the inputs of the NDI scanner hardware 22. In otherwords, the software running on the motion tracking and applicationscomputer 16 converts location data into simulated quadrature pulseinstructions and sends those pulse commands (via a USB connection) tothe data acquisition device 20 that generates the simulated quadraturepulse signals.

The simulated X and Y pulse signals are received by an NDI processor ofthe NDI scanner hardware 22. The NDI processor decodes the simulated Xand Y quadrature pulse signals into X-Y position data representing theposition of the NDI sensor unit 8 in two dimensions. The NDI processorof the NDI scanner hardware 22 also receives NDI scan imaging data fromthe NDI sensor 8 via cable 11. The 2-D NDI scan data is then mapped tothe 3-D space of the model of the fuselage section using the X,Y,Zcoordinates of the location of the end effector from either the motioncapture data (described above) or encoder data and forward kinematicscomputation (described later). Visualization of the 2-D NDI scan data ina 3-D environment may use texture maps projected onto the surface of the3-D solid models.

In accordance with one embodiment, the NDI scanner hardware 22 is anintegrated unit comprising a pulser, a receiver and an electronics box.An NDI processor that is part of the receiver converts the simulatedencoder pulses into a current X,Y position in accordance with theformulas:X_pos=num_of_x_pulses_received*x_scale_factorY_pos=num_of_y_pulses_received*y_scale_factorwhere each scale factor is a small number (e.g., on the order of 0.01inch per pulse). This X,Y position is updated many times per second.

At the same time the NDI sensor unit 8 is capturing inspection data(e.g., scan imaging data). In the case where ultrasonic detection isbeing utilized, data from each element of a linear array of ultrasonicelements can be acquired. These elements make up an array that isanalogous to a row of pixels on a computer monitor (where each row isoffset from the next by defining the starting X,Y position of the firstpixel and when all the rows are displayed in proper order, a full imagecan be displayed).

Each time the array is moved a predefined distance, the NDI scannerhardware 22 receives a new strip of scan data from the NDI sensor unit 8(representing a “row” of pixel data) via cable 11. Each strip of scandata is saved to memory. The NDI software uses the current X_pos andY_pos data derived from the received pulse data to locate the startingpoint in the image where to place the row of pixels.

The NDI scanner hardware 22 associates the position data and the scanimaging data. The associated data is then sent to an NDI analysis anddisplay computer 24, which uses the position data and scan imaging datato assemble a two-dimensional image of the object being scanned fordisplay on display monitor 26. In accordance with one implementation,the NDI scanner hardware 22 and the NDI analysis and display computer 24may be connected components of a phased array acquisition instrumentsuch as the TomoScan FOCUS LT, which is commercially available fromOlympus Corporation.

The position data derived from the simulated encoder pulses enables theNDI analysis and display computer 24 to compute and display a finalC-scan image. The C-scan presentation provides a plan-type view of thelocation and size of part features. In the case of ultrasound imaging,the plane of the image is parallel to the scan pattern of the ultrasonictransducer array of the NDI sensor unit. In a C-scan, there is locationinformation shown in the display. The location is found along thehorizontal and vertical axes (or rulers) of the NDI data display.Individual pixels make up the C-scan. The width of each pixel directlycorresponds to a specific number of pulses, which is defined by theresolution of a simulated dimensional encoder associated with a firstaxis of the part, while the height of each pixel directly corresponds toa specific number of pulses, which is defined by the resolution of asimulated dimensional encoder associated with a second axis of the partwhich is perpendicular to the first axis. Operators are able to makearea measurements of flaws that might show up in the C-scan.

In an alternative display mode, the 3-D model of the fuselage section,the NDI sensor unit location data, and the scan data can be used togenerate the three-dimensional image graphically represented in FIG. 3.

FIG. 5 shows one possible configuration for the system, but otherconfigurations are also possible, such as having the motion capture andapplication software running on a single computer, or combining the NDIscanning system components into a single device. More details concerningthe motion capture process can be found in U.S. patent application Ser.No. 13/744,730.

In accordance with alternative embodiments, the location of the NDIsensor unit 8 (see FIG. 4) can be tracked using positional encoders onlythat measure incremental movements of: (1) the arch frame 50 along thelinear rails; (2) the carriage 52 along the curved tracks of the archframe 50; and (3) the inner sleeve 66 of the extendible arm 62 relativeto the outer sleeve 64. In accordance with one OML scanningimplementation, the encoder-based solution (no motion capture) usesthree separate linear encoders.

FIG. 6 is a block diagram showing components of a control system thatuses rotational or linear encoders to track the relative location (e.g.,relative to an initial location acquired using a local positioningsystem) of an NDI sensor unit mounted to an external scanning system ofthe type partly depicted in FIG. 1. The control system comprises aground-based computer 150 programmed with motion control applicationsoftware 152 and NDI scan application software 154. The control computer150 is connected to an X-axis motion motor 160 (which drives translationof the arch gantry system 30 along the linear rails), a radial sweep(i.e., Y-axis motion) motor 164 (which drives circumferential movementof carriage 52 along the curved tracks of the arch gantry system 30),and an extension (i.e., Z-axis motion) motor 168 (which drivesextension/retraction of the extendible arm 62). The control computer 150may comprise a general-purpose computer programmed with motion controlapplication software 152 comprising respective software modules forcontrolling the motors. The motion control application 152 controls theoperation of the motors based on position feedback from respectiveencoders, namely, X-axis encoder 162, Y-axis encoder 166, and Z-axisencoder 170.

The control computer 150 is connected to the motors and encoders via anelectronics box (not shown in FIG. 6). The electronic box contains thesystem power supplies and a data acquisition device 20, and integratesall the scanner control connections and provides an interface betweenthe control computer 150 and respective flexible electrical cables thatconnect to the gantry, carriage and extendible arm.

Motion control application software 152 controls the extension motor 168to produce specified radial motion of the UT arrays 73. The range ofradial motion of the UT arrays 73 in both directions may be limited bylimit switches (not shown). In accordance with one embodiment, theZ-axis encoder 170 measures the angular position of the output shaft ofan extension motor 168 that drives rotation of a lead screw, whichangular position in turn determines the radial displacement of theultrasonic transducer arrays 73 effectuated by the extendible arm 62.The motion control application software 152 is thus capable of movingthe UT array 73 circumferentially along the OML of the fuselage sectionbeing inspected.

In accordance with one embodiment, the encoded data from linear encoders162, 166 and 170 is received by a data acquisition device 20. The dataacquisition device 20 also has digital input and output connections thatare used for multiple functions within the system.

Direct use of the X and Y encoder data by the NDI software will give arough approximation of the surface motion, but not a perfect solution,since the fuselage section will not be perfectly circular.Alternatively, it is possible to use kinematics equations to get anaccurate estimation of the surface X-Y translation, assuming thecomponents are not too flexible. The kinematics equations take intoaccount the non-circular shape of the fuselage section. Using thisapproach requires that the data acquisition device 20 generate simulatedencoder pulses. In order to calibrate the encoder-based system, all ofthe encoders would need to be zeroed in a known location relative to thefuselage section. The data acquisition device may not be necessary forsome LPS-plus-encoder embodiments where the fuselage section iscircular.

The data acquisition device 20 can be configured to generate simulatedquadrature encoder pulses representing incremental Y motion along theOML of the fuselage section. Those simulated encoder pulses are outputthrough digital output ports to an ultrasonic pulser/receiver 156. Theultrasonic pulser/receiver 156 also receives pulses generated by theX-axis encoder 162. The pulser/receiver 156 sends the encoder pulses tothe NDI scan application 154. The NDI scan application 154 uses theencoder values to position the scan data in the proper location.

The control computer 150 hosts ultrasonic data acquisition and displaysoftware that controls the ultrasonic pulser/receiver 156. Theultrasonic pulser/receiver 156 in turn sends pulses to and receivesreturn signals from the ultrasonic transducer arrays 73. The NDI scanapplication software 154 controls all details of the scan data and thedisplay of data.

Referring to FIGS. 1 and 2, an exemplary process for externally scanninga fuselage section will now be described. First, the fuselage section 12is moved into position under the arch gantry system 30. The arch gantrysystem 30, carriage 52 and extendible arm 62 should be in theirrespective starting positions. The motion capture system is turned onand correlated to the retro-reflective markers 10 attached to the scanshoe of the NDI sensor unit 8 and to the retro-reflective targets 36which are temporarily attached to the fuselage section 12. The radialmotion actuator is activated so that the NDI sensor unit 8 is movedcircumferentially from one side edge of a half-barrel fuselage section12 to the other side edge. A wide swath of ultrasound scan data, that isslightly less than the width of the multi-array shoe, is collected andstored for analysis. During scanning, the motion capture cameras 2directed toward the retro-reflective markers 10 precisely track themotion and data location in 3-D space. The ultrasonic scan data is thencorrelated to a 3-D model of the fuselage section (as shown in FIG. 3)for efficient processing by an analyst. Once the full half-circumferenceis scanned, the NDI sensor unit 8 is lifted from the external surface ofthe fuselage section and, in one embodiment, returned to its startingposition. In another embodiment, the NDI sensor unit 8 is shifted overand returns on an adjacent path. In addition, the arch gantry system 30is moved axially along the linear rails to an adjacent location(displaced by the width of the circumferentially scanned area from theprevious location). The radial motion actuator is re-activated tocollect the adjacent swath of scan data, and this is repeated down thelength of the half-barrel fuselage section 12 until the scan iscompleted. If a full-barrel fuselage section is being scanned, thefuselage section is rotated 180 degrees about the longitudinal (roll)axis and the process is repeated.

The benefits of the external scanning system disclosed above includefast scanning, simple programming, a simple structure, and lowmaintenance costs. Real-time tracking of the scanner also enablesreal-time visual indication of the path displayed on a 3-Drepresentation of the half-barrel fuselage section with path tracelines. The moving arch gantry system can also be displayed in real-time.Locations of potential problem areas could be marked visually (and notedin the coordinate system of the fuselage section). Scan plans could bepreviewed on the 3-D model to confirm the motion plan before startingthe actual scan. Also, the control system could be set up to allow theuser to select a point on the 3-D model of the full or half barrel andhave the arch gantry system move to the selected location.

In addition to scanning the outer mold line of the fuselage section, thestringers of stiffened fuselage sections can be concurrently (or at adifferent time) scanned using a mobile scanner platform disposed insidethe fuselage section. Inspecting hat stringers (i.e., stringerscomprising a straight central cap portion, two angled sides connected tothe central cap portion by respective cap corners, and two flangesconnected to the angled sides by respective flange corners) normallyrequires a one-sided inspection technique, such as pulse echo ultrasonic(PEU) inspection. The NDI sensor unit can be configured and thenstrategically placed and oriented to ensure full inspection of theentire hat stringer in one pass. Support structures for inspectionsensors, also referred to as shoes, may be fabricated for specificplacement and orientation of UT arrays corresponding to the intendedshapes and sizes of hat stringers.

In many cases, each hat stringer is a trapezoidal structure comprisingangled sides which connect to a cap at respective cap corners. Each hatstringer is affixed to the skin of the fuselage section by respectiveflanges which connect to the angled sides of the hat stringer atrespective flange corners. In order to inspect hat stringers having sucha structure, one approach is known using a suite of seven transducerarrays: one to inspect a central cap portion; two to inspect angledsides; two to inspect cap corners; and two to inspect flange corners. Itshould be understood that the term “corner” as used herein refers to aradiused (i.e., filleted) intersection of surfaces. The central capportion may be a planar surface connecting the cap corners.

In accordance with the teachings herein, the NDI sensor unit forinternal scanning can be designed and configured for inspecting hatstringers, stringers having rounded caps, or stringers having otherprofiles. For example, for inspecting a hat stringer, the NDI sensorunit may comprise seven UT arrays, whereas for inspecting a rounded capstringer, only five UT arrays can be employed, as disclosed in U.S.patent application Ser. No. 13/975,599. In either case, the NDI sensorunit can be coupled to the distal end of a pivotable telescoping arm ofa computer-controlled manipulator for scanning each stringer in alengthwise direction (assuming, for the purpose of illustration, thatthe stringer is straight).

FIG. 7A is a diagram representing an end view of portions of an internalscanning system for scanning stringers 48 of a half-barrel fuselagesection 12 in accordance with one embodiment. The side edges of thehalf-barrel fuselage section 12 are supported by left and right supports74 and 76 respectively. Although supports 74 and 76 are depicted asfixed structures, in the alternative the fuselage section may betransported into position for inspection by means of automated guidedvehicles or other transport devices, with the understanding that thefuselage section is stationary during the internal scanning operation.

The internal scanning system depicted in FIG. 7A comprises an NDI sensorunit 100 coupled to a distal end of an extendible arm 90 which ispivotably coupled to a holonomic motion platform 80. A holonomic systemis one that is not subject to motion constraints. As used in thisdisclosure, a vehicle is considered to be holonomic if the controllabledegrees of freedom are equal to the total degrees of freedom. This typeof system can translate in any direction while simultaneously rotating.This is different than most types of ground vehicles, such as car-likevehicles, tracked vehicles, or wheeled differential-steer (skid-steer)vehicles, which cannot translate in any direction while rotating at thesame time.

In accordance with the embodiment depicted in FIG. 7A, the holonomicmotion platform 80 comprises a base 84, four omni (or Mecanum) wheels 82(only two of which are visible in FIG. 7A) rotatably coupled to the base84, four motors (not shown, but mounted to the base 84) whichrespectively actuate rotation of the wheels; a frame 86 attached to thebase 84, and a pivot joint 88 supported the frame 86 and connected tothe extendible arm 90. The motors onboard the holonomic motion platform80 can be controlled initially by a system operator during set-up andlater by motion control software during scanning. The holonomic motionplatform 80 may be controlled to move parallel to a projection of thestringer axis onto the ground. In cases wherein the profile of thefuselage section is constant, the stringers will be parallel to theX-axis (see FIG. 1). In these cases, the NDI sensor unit 100 will travelalong the stringer being inspected as the holonomic motion platform 80is moved in the X-axis direction. The system seen in FIG. 7A furthercomprises an encoder wheel 98 rotatably coupled to the NDI sensor unit.As the NDI sensor unit 100 scans the stringer and acquires scan data, anencoder (not shown) coupled to the encoder wheel 98 outputs encoderpulses which will be used to correlate the scan data with an axialposition of the origin of the NDI sensor unit.

Still referring to FIG. 7A, the extendible arm 90 comprises a pivotableouter sleeve 92 connected to the pivot joint 88, an inner sleeve 94 thatis translatable inside the outer sleeve 92, and an NDI sensor unit 100coupled to the inner sleeve 94 by means of a compliant support structuresimilar to the compliant support structure 70 previously described indetail with reference to FIG. 4. Such a compliant support structureenables the NDI sensor unit 100 to adjust its radial position inresponse to variations in the size and shape of the stringer 48 as theNDI sensor unit 100 travels along the length of the stringer. Suchcompliant motion is indicated by a double-headed arrow in FIG. 7A.

In addition, a counterweight 96 is coupled to one end of the outersleeve 92 of the extendible arm 90. Optionally, the counterweight 96 maybe movable back and forth along the outer sleeve 92 for the purpose ofbalancing the moments on opposite sides of the pivot joint 88 to achievea boom arm equilibrium position. As disclosed in U.S. patent applicationSer. No. 14/176,169 entitled “Automated Mobile Boom System for CrawlingRobots”, the counterweight 96 could be moved by a motor-driven,non-backdrivable lead screw 46 that holds the counterweight in placeeven when power is disrupted. Control of counterweight position could beprovided either by direct operator commands or by a computer programmedin accordance with an automatic balancing algorithm. The automatedposition control is based on feedback of the tilt angle in order toachieve a neutrally balanced telescoping arm. The counterweight motionrate would be sufficient to address static balance or slow changes tothe balance point.

To set up the internal scanning system, first the holonomic motionplatform 80 is driven to starting location (i.e., position andorientation) on the floor. Then a first actuator (not shown) isactivated to pivot the extendible arm 90 to a target tilt angledependent on the angular position of the stringer to be inspected. (Thepivotability of the extendible arm 90 is indicated by a double-headedarrow labeled “Actuator 1” in FIG. 7A.) At the target tilt angle, theinner sleeve 94 of the extendible arm 90 can be extended by a secondactuator (not shown) until the NDI sensor unit 100 engages the stringerto be inspected. (The extendability of the extendible arm 90 isindicated by a double-headed arrow labeled “Actuator 2” in FIG. 7A.)Respective encoders can be coupled to the first and second actuators toprovide location feedback to the motion control software.

In the embodiment shown in FIG. 7A, the location of the NDI sensor unit100 can be tracked using a motion capture system of the type previouslydescribed with reference to FIG. 5. The motion capture system partlydepicted in FIG. 7A comprises a plurality of retro-reflective markers 35attached the base 84 of the holonomic motion platform 80, a plurality ofretro-reflective markers 36 attached (temporarily) to the half-barrelfuselage section 12, and a plurality of retro-reflective markers 38attached to the inner sleeve 94 of the extendible arm 90. Alternatively,retro-reflective markers 38 may be attached directly to the end effectoror NDI sensor unit 100. A motion capture processor (not shown in FIG.7A) is programmed to compute the respective locations of the holonomicmotion platform 80 and extendible arm 90 in the frame of reference ofthe fuselage section 12. The motion capture system provides continuousabsolute coordinate results. Note that for the embodiments that includemotion capture (e.g., FIG. 7A), encoders are not required for the basicconcept. But, an optional encoder coupled to encoder wheel 98 on theend-effector (as shown in FIG. 7A) may be used if higher resolution isdesired.

When the NDI sensor unit 100 is located correctly relative to thestringer to be inspected, the NDI sensor unit is activated. Then theholonomic motion platform 80 is moved in the X-axis direction, causingthe NDI sensor unit 100 to travel along the stringer. A swath ofultrasound scan data is acquired as the NDI sensor unit scans over theradii and flats of the stringer and stored for analysis. Small motionsof the NDI sensor unit 100 that would not be possible to accuratelydetect with measurements taken only at the base 84 of the holonomicplatform 80 are tracked using the encoder wheel 98, which rotates as theNDI sensor unit 100 travels along the length of the stringer 48 duringscanning. For higher resolution, an encoder (not shown) coupled to theencoder wheel 98 outputs encoder pulses that are used to correlate theultrasound scan data with the axial position of the origin of the NDIsensor unit 100.

Once the entire length of a stringer at a particular radial angle hasbeen scanned, the extendible arm 90 is rotated to the angular positionof the next stringer and the above steps are repeated. This procedure isfollowed until all of the stringers have been scanned.

FIG. 7B is a diagram representing an end view of portions of an internalscanning system for scanning stringers of a half-barrel fuselage sectionthat uses LPS tracking instead of motion capture tracking. In accordancewith this embodiment, the NDI sensor unit 100 is coupled to a distal endof an extendible arm 90 which is pivotable relative to a holonomicmotion platform 80. This hardware configuration may be identical to thatdescribed with reference to FIG. 7A.

In the embodiment depicted in FIG. 7B, three calibration points (i.e.,optical targets) 102 a-102 c are temporarily attached to the fuselagesection 12, while another three calibration points 104 a-104 c areattached to the holonomic motion platform 80. These calibration pointscan be detected using an LPS (not shown in FIG. 7B) of the typedisclosed in U.S. Pat. No. 7,859,655. Such an LPS permits an operator toacquire local coordinate measurement and imaging data for an object inthe field of view of the physical hardware of the LPS. The LPS may use apan-tilt unit to orient the instrument (a camera and laser range meter)in the direction of a target object for which local coordinates areneeded. The laser range meter can be used to measure range to theobject, or distances can be entered or derived algorithmically. Imagedata, measured range data, and pan-tilt angles are used along with knowncalibration points to determine the location of the LPS device relativeto the target object. With the relative location known, additional LPSmeasurements are converted into the local coordinates of the targetobject's coordinate system (such as the coordinate system of thefuselage section 12). More specifically, the LPS disclosed in U.S. Pat.No. 7,859,655 can use known calibration points 102 a-102 c on thehalf-barrel fuselage section 12 to perform an instrument-to-targetcalibration, after which the LPS can be used to measure the coordinatesof the calibration points 104 a-104 c on the holonomic motion platform80 in the coordinate system of the fuselage section 12. More detailsconcerning the operation of an LPS will be provided later with referenceto FIG. 15.

The LPS can be used to take discrete measurements at specific times(such as during initialization), while encoders are used to getcontinuous data. For example, during the initial setup, the LPS is usedto determine the position and orientation of the base 84 of theholonomic motion platform 80 with respect to the fuselage section 12.After that, the relative motion data from the encoders are used alongwith that initial position and orientation data to provide acontinuously updated position and orientation of the NDI sensor unit100. Note that LPS may not be fast enough to capture the measurementdata continuously. More importantly, small motions at the NDI sensorunit 100 are captured by the encoder data that would not be possible toaccurately detect with measurements taken only at the base 84.

FIG. 8 is a diagram representing an end view of portions of an internalscanning system for scanning stringers 48 of a half-barrel fuselagesection 12, such system having a double arm configuration (seeextendible arms 90 a and 90 b) and comprising a holonomic motionplatform 80 with LPS tracking. In some embodiments, the outer sleeves 92of the extendible arms 90 a and 90 b may have a fixed relationship toeach other and may be rotatable together with a single actuator, asindicated by the double-headed arrow labeled “Actuator 1”. In otherembodiments the two arms may be able to rotate separately usingindependent actuators. The inner sleeves 94 of the extendible arms 90 aand 90 b are independently translatable relative to the respective outersleeves 92, as indicated by the double-headed arrows respectivelylabeled “Actuator 3” and “Actuator 2” in FIG. 8. The double armconfiguration depicted in FIG. 8 enables two NDI sensor units 100 toscan respective stringers 48 concurrently as the holonomic motionplatform 80 travels in an X-axis direction.

FIG. 9 shows an end view of portions of an internal scanning system forscanning stringers of a half-barrel fuselage section, such system havingthe same double arm configuration seen in FIG. 8, but comprising alinear motion platform 78 whose location is tracked using an LPS. Thelinear motion platform 78 comprises linear bearings 106 a and 106 bwhich respectively ride on floor-mounted linear rails 110 a and 110 bwhich are parallel to the X-axis direction. As the linear motionplatform 78 moves along the X-axis, the NDI sensor units 100 scanrespective stringers 48. The instantaneous X-position of each NDI sensorunit can be tracked by respective encoders coupled to respective encoderwheels 98 which roll along respective surfaces of the stringers beinginspected. That encoder data is then used to correlate the X-position ofeach NDI sensor unit to the scan data acquired.

FIG. 10 is a diagram representing an end view of portions of an internalscanning system for scanning stringers 48 of a half-barrel fuselagesection 12, such system having a single-arm, double-end-effectorconfiguration in accordance with an alternative embodiment. This systemcomprises a linear motion platform 78 with LPS tracking.

In accordance with the embodiment depicted in FIG. 10, the linear motionplatform 78 comprises a base 84, linear bearings 106 a and 106 b whichrespectively ride on floor-mounted linear rails 110 a and 110 b, twomotors (not shown, but mounted to the base 84) which respectivelyactuate translation of the platform along the linear rails 110 a and 110b, a frame 86 attached to the base 84, and a pivot joint 88 supported bythe frame 86 and connected to the extendible arm 90.

Still referring to FIG. 10, the extendible arm 90 comprises a pivotableouter sleeve 92 connected to the pivot joint 88, an inner sleeve 94 thatis translatable inside the outer sleeve 92, a transverse member 114attached to a distal end of the inner sleeve 94, and a pair of NDIsensor units 100 a and 100 b coupled to the transverse member 114 bymeans of respective compliant support structures similar to thecompliant support structure 70 previously described in detail withreference to FIG. 4. Such compliant support structures enable each NDIsensor unit to adjust its radial position in response to variations inthe size and shape of the corresponding stringer 48 as the NDI sensorunit travels along the length of a stringer. Such compliant motion isindicated by double-headed arrows in FIG. 10.

FIG. 11 shows an end view of portions of an internal scanning systemhaving a single-arm, double-end-effector configuration in accordancewith an alternative embodiment. This embodiment differs from theembodiment shown in FIG. 10 in that, instead of a transverse memberaffixed to a distal end of the extendible arm 90, a pivotable crossmember 116 is pivotably coupled to the distal end of the extendible arm90. The pivotable coupling preferably includes means for spring-loadedcompliant pivoting motion of cross member 116 (which compliant motion isin addition to the compliant motion provided by the compliant supportstructures which couple the NDI sensor units 100 a and 100 b to thecross member 116).

FIGS. 12 and 13 are diagrams representing respective end views ofportions of an internal scanning system for scanning stringers of afull-barrel fuselage section 112 supported by a support base 113. Thissystem has a single-arm, double-end-effector configuration of the typeshown in FIG. 10 and a linear motion platform 78. The linear motionplatform 78 comprises rollers 132 that ride on a pair of linear rails130 a and 130 b. In this case the linear rails 130 a and 130 b aremounted to and supported by a platform lift 120 instead of on the floor.

The linear motion platform 78 is shown in respective locations forscanning stringers 48 on the upper half (FIG. 12) and lower half (FIG.13) of the full-barrel fuselage section 112. The platform lift 120comprises a pair of extendible supports which support a (see bridge beam136 in FIG. 14) having a length greater than the length of the fuselagesection being inspected. Each extendible support comprises a pedestal122, a fixed outer sleeve 124 projecting vertically upward from thepedestal 122, an inner sleeve 126 that is translatable inside the outersleeve 124, and a bearing 128 mounted to a distal end of the innersleeve 126.

FIG. 14 shows a top view of some components of the system depicted inFIGS. 12 and 13. The dashed rectangle indicates the longitudinal andtransverse dimensions of a full-barrel fuselage section 112 surroundinglinear rails 130 a and 130 b and a bridge beam 136 of the platform lift.The ends of the linear rails 130 a and 130 b are respectively connectedto respective cross beams 134 which are attached to and projectlaterally from the bridge beam 136 on both sides thereof. The linearmotion platform (not shown in FIG. 14) can travel from one end of linearrails 130 a and 130 b to the other end.

In order to place a full-barrel fuselage section 112 in the positionseen in FIGS. 12 and 13, one end of the platform lift 120 would need tobe disassembled while the corresponding end of the bridge beam 136 issupported by other means.

In the configuration shown in FIG. 12, the linear motion platform 78 isupright and the pivot joint 88 is located at a center of the full-barrelfuselage section 112. In this configuration, the stringers 48 attachedto the upper half of the full-barrel fuselage section 112 can bescanned.

In the configuration shown in FIG. 13, the supports are extended and thelinear motion platform 78 is rotated 180 degrees. The supports areextended by a distance such that the pivot joint 88 is again located atthe center of the full-barrel fuselage section 112 when the linearmotion platform 78 is upside down. In this configuration, the stringers48 attached to the lower half of the full-barrel fuselage section 112can be scanned.

In each of the embodiments depicted in FIGS. 8 through 12, the LPS canbe used to take discrete measurements at specific times (such as duringinitialization), while encoders are used to get continuous data. Inaccordance with respective alternative embodiments, a motion capturesystem can be used to track the location of the NDI sensor unit.

FIG. 15 shows a physical setup in which an LPS is used to track alocation of a linear motion platform 78 relative to a half-barrelfuselage section 12. The LPS comprises a single video camera 140 and alaser range meter (not shown) on a controllable pan-tilt mechanism 142with angle measurement capability mounted on a tripod 144. The videocamera 140 may have automated (remotely controlled) zoom capabilities.The video camera 140 may additionally include an integral crosshairgenerator to facilitate precise locating of a point within an opticalimage field display of the video camera. The video camera 140 andpan-tilt mechanism 142 may be operated by an LPS computer 148. The LPScomputer 148 communicates with the video camera 140 and the pan-tiltmechanism 142 through a video/control cable 146. Alternatively, the LPScomputer 148 may communicate with video camera 140 and pan-tiltmechanism 142 through a wireless communication pathway. The pan and tiltangles of the pan-tilt mechanism 142 and, therefore, the orientation ofthe video camera 140 can be controlled using the keyboard of the LPScomputer 148 or other input device, such as the gamepad interface 172shown in FIG. 15. The optical image field, with crosshair overlay, assighted by the video camera 140, can be displayed on the monitor 174 ofthe LPS computer 148.

The pan-tilt mechanism 142 is controlled to positionally adjust thevideo camera 140 to selected angles around a vertical, azimuth (pan)axis and a horizontal, elevation (tilt) axis. A direction vector 138,that describes the orientation of the camera 140 relative to the fixedcoordinate system of the tripod 144 (or other platform on which thepan-tilt unit is attached), is determined from the pan and tilt angles,as well as the position of the center of a crosshair marker in theoptical field when the camera 140 is aimed at a point of interest. Thisdirection vector 138 is depicted in FIG. 15 as a dashed line extendingfrom the lens of the camera 140 and intersecting a calibration point 102a on the fuselage section 12. The other calibration points 102 b and 102c on the fuselage section and the calibration points 104 a-104 c on thelinear motion platform 78 will each be targeted in turn and the datathus acquired can be processed by the LPS computer 148 to calculate theposition and orientation offset of the linear motion platform 78relative to the fuselage section 12.

A laser range meter may be incorporated inside the housing of camera 140or mounted to the outside of camera 140 in such a way that it transmitsa laser beam along the direction vector 138. The laser range meter isconfigured to measure the distance to each calibration point. The laserrange meter may have a laser and a unit configured to compute distancesbased on the laser light detected in response to a laser beam reflectedby the each calibration point.

The local positioning system shown in FIG. 15 further comprisesthree-dimensional localization software which is loaded into the LPScomputer 148. For example, the three-dimensional localization softwaremay be of a type that uses multiple non-collinear calibration points 102a-102 c on the fuselage section 12 to define the location (position andorientation) of video camera 140 relative to the fuselage section 12.Calibration points 102 a-102 c can be temporarily attached to thefuselage section 12. Alternatively, features on the fuselage section 12can be used as calibration points.

The measured distances to the calibration points 102 a-102 c may be usedin coordination with the pan and tilt angles from the pan-tilt mechanism142 to solve for the camera position and orientation relative to thefuselage section 12. A method for generating an instrument to targetcalibration transformation matrix (sometimes referred to as the camerapose) is disclosed in U.S. Pat. No. 7,859,655. Using the measured data,the calibration process computes the 4×4 homogeneous transformationmatrix that defines the position and orientation of the video camera 140(and laser range meter) relative to the fuselage section 12.

The LPS seen in FIG. 15 can also be used to determine the position andorientation of the video camera 140 relative to base 84 of the linearmotion platform 78. The LPS uses multiple non-collinear calibrationpoints 104 a-104 c on the base 84 to define the location (position andorientation) of video camera 140 relative to the base 84. Given thelocation of the fuselage section 12 relative to the video camera 140 andthe location of the base 84 relative to the video camera 140, thelocation of the base 84 relative to the fuselage section 12 can bedetermined at the start of each work sequence.

During each work sequence, a motion controller 180 can control the base84 to move in a manner that allows the end effectors 100 to scanrespective stiffeners 48 of the fuselage section 12. The LPS computer148 communicates with the motion controller 180 through a cable 178; themotion controller communicates with actuators (not shown) on the base 84through a cable 182. The motion controller 180 is preferably a computerprogrammed with motion control software. The motion controller 180 willbe able to control the linear motion of the linear motion platform 78,as well as the location of the end effector 100 of arm 90 a (and 90 b)based on the starting location (position and orientation) of base 84relative to fuselage section 12 as calculated by the LPS computer 148.

Using the foregoing methodology, the initial location of a movablescanning platform relative to a stationary fuselage section can bedetermined. The scanning platform is then moved in the X direction whilethe first stringer is scanned. During a scan in the X direction, thedisplacement of the scanning platform relative to its starting positioncan be tracked using an encoder wheel mounted to an end effector beingcarried by the scanning platform. Because the starting X position of theend effector relative to the fuselage section is known, the varying Xposition of the end effector (defined in the coordinate system of thefuselage section) can be tracked. When the end effector reaches the endof the first stringer, the scanning platform can be returned to itsstarting position, moved to the next stringer location, and then theprocess can be repeated for the next stringer to be inspected.

The benefits of the internal scanning system disclosed above includescanning at a high rate sufficient to maintain production schedules. Thescanner does not have to be repositioned by a human operator for eachstringer; instead the mechanism can be programmed to move to the nextstringer automatically. Other benefits include simple programming,simple structure, and reduced maintenance costs. The system is modularand can be moved to a new location easily, especially the holonomicmotion version.

The systems and methods disclosed above may be employed in an aircraftmanufacturing and service method 200 as shown in FIG. 16 for inspectingparts of an aircraft 202 as shown in FIG. 17. During pre-production,exemplary method 200 may include specification and design 204 of theaircraft 202 and material procurement 206. During production, componentand subassembly manufacturing 208 and system integration 210 of theaircraft 202 takes place. Thereafter, the aircraft 202 may go throughcertification and delivery 212 in order to be placed in service 214.While in service by a customer, the aircraft 202 is scheduled forroutine maintenance and service 216 (which may also includemodification, reconfiguration, refurbishment, and so on).

Each of the processes of method 200 may be performed or carried out by asystem integrator, a third party, and/or an operator (e.g., a customer).For the purposes of this description, a system integrator may includewithout limitation any number of aircraft manufacturers and major-systemsubcontractors; a third party may include without limitation any numberof venders, subcontractors, and suppliers; and an operator may be anairline, leasing company, military entity, service organization, and soon.

As shown in FIG. 17, the aircraft 202 produced by exemplary method 200may include an airframe 218 (comprising, e.g., a fuselage, frames,stiffeners, wing boxes, etc.) with a plurality of systems 220 and aninterior 222. Examples of high-level systems 220 include one or more ofthe following: a propulsion system 224, an electrical system 226, ahydraulic system 228, and an environmental control system 230. Anynumber of other systems may be included. Although an aerospace exampleis shown, the principles disclosed herein may be applied to otherindustries, such as the automotive industry.

Apparatus and methods embodied herein may be employed during any one ormore of the stages of the production and service method 200. Forexample, components or subassemblies fabricated or assembled duringproduction process 208 may be inspected using the inspection systemdisclosed herein. Also, one or more apparatus embodiments, methodembodiments, or a combination thereof may be utilized during theproduction stages 208 and 210, for example, by substantially expeditingassembly of or reducing the cost of an aircraft 202. Similarly, one ormore of apparatus embodiments, method embodiments, or a combinationthereof may be utilized while the aircraft 202 is in service, forexample and without limitation, during maintenance and service 216.

While NDI scanning systems have been described with reference to variousembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the teachingsherein. In addition, many modifications may be made to adapt theteachings herein to a particular situation without departing from thescope thereof. Therefore it is intended that the claims not be limitedto the particular embodiments disclosed herein.

As used in the claims, the term “computer system” should be construedbroadly to encompass a system having at least one computer or processor,and which may have multiple computers or processors that communicatethrough a network or bus. As used in the preceding sentence, the terms“computer” and “processor” both refer to devices having a processingunit (e.g., a central processing unit) and some form of memory (i.e.,computer-readable medium) for storing a program which is readable by theprocessing unit. For example, a computer system may comprise respectiveprocessors incorporated in a plurality of devices and a control computerin communication with those processors.

As used in the claims, the term “location” comprises position in a fixedthree-dimensional coordinate system and orientation relative to thatcoordinate system.

The method claims set forth hereinafter should not be construed torequire that the steps recited therein be performed in alphabeticalorder (alphabetical ordering in the claims is used solely for thepurpose of referencing previously recited steps) or in the order inwhich they are recited. Nor should they be construed to exclude two ormore steps or portions thereof being performed concurrently or toexclude any portions of two or more steps being performed alternatingly.

The invention claimed is:
 1. A method for scanning a stringer in aninside of a workpiece having a curved section that extendslongitudinally and circumferentially, the method comprising: placing aworkpiece having a curved section that extends longitudinally andcircumferentially so that the workpiece is stationary relative to afloor; moving a mobile platform to a starting position on the floor andunderneath the workpiece; pivoting an extendible arm that is pivotablycoupled to the mobile platform to a target tilt angle dependent on anangular position of a stringer to be inspected; at the target tiltangle, extending an inner sleeve of the extendible arm until a first NDIsensor unit coupled to a distal end of the inner sleeve engages a firstportion of the stringer to be inspected; activating the first NDI sensorunit to inspect the first portion of the stringer; processing signalsoutput from the first NDI sensor unit to derive first scan datacharacterizing a structural state of the first portion of the stringer;and acquiring first location data representing a first location of thefirst NDI sensor unit when the first NDI sensor unit engages the firstportion of the stringer.
 2. The method as recited in claim 1, furthercomprising: moving the mobile platform in a manner that causes the firstNDI sensor unit to travel along the stringer until the first NDI sensorunit engages a second portion of the stringer; activating the first NDIsensor unit to inspect the second portion of the stringer; processingsignals output from the first NDI sensor unit to derive second scan datacharacterizing a structural state of the second portion of the stringer;and acquiring second location data representing a second location of thefirst NDI sensor unit when the first NDI sensor unit engages the secondportion of the stringer.
 3. The method as recited in claim 2, furthercomprising: moving a curved track along first and second linear tracksto a first longitudinal position, the curved track in the firstlongitudinal position being disposed radially outward from the curvedsection of the workpiece; adjusting a position of a second NDI sensorunit relative to a carriage to which the second NDI sensor unit isadjustably coupled by an extendible arm, the carriage in turn beingmovable along the curved track; moving the carriage along the curvedtrack while the curved track is stationary at the first longitudinalposition; activating the second NDI sensor unit to inspect astrip-shaped area of the curved section of the workpiece as the carriageis moved along the curved track; and processing signals output from thesecond NDI sensor unit to derive a strip of scan data characterizing astructural state of the strip-shaped area of the curved section of theworkpiece.
 4. The method as recited in claim 3, wherein the second NDIsensor unit is activated while the first NDI sensor unit is moving alongthe stringer.
 5. The method as recited in claim 1, wherein a pathfollowed by the mobile platform during moving is parallel to aprojection of an axis of the stringer onto the ground.
 6. The method asrecited in claim 1, further comprising adjusting a radial position ofthe first NDI sensor unit as the first NDI sensor unit travels along thestringer.
 7. The method as recited in claim 1, wherein the mobileplatform is a holonomic motion vehicle.
 8. The method as recited inclaim 1, wherein the mobile platform moves on tracks.
 9. The method asrecited in claim 1, wherein the workpiece is a fuselage section.
 10. Themethod as recited in claim 9, wherein the stringer is made of compositematerial.
 11. The method as recited in claim 1, wherein the first scandata is acquired from at least a central cap portion of the stringer andan angled side of the stringer.
 12. A system for supporting a mobileplatform inside a curved cylinder-like workpiece, comprising: a firstextendible support comprising a first pedestal, a first fixed outersleeve projecting vertically upward from said first pedestal, a firstinner sleeve that is translatable inside said first outer sleeve, and afirst bearing mounted to a distal end of said first inner sleeve; asecond extendible support comprising a second pedestal, a second fixedouter sleeve projecting vertically upward from said second pedestal, asecond inner sleeve that is translatable inside said second outersleeve, and a second bearing mounted to a distal end of said secondinner sleeve; a bridge beam spanning a space between said first andsecond extendible supports, said bridge beam having a first endrotatably coupled to said distal end of said first inner sleeve by saidfirst bearing and a second end rotatably coupled to said distal end ofsaid second inner sleeve by said second bearing; first and second crossbeams attached to and projecting laterally from said bridge beam; firstand second linear rails which are attached to said first and secondcross beams in a mutually parallel relationship; and a mobile platformcomprising a frame, a first set of rollers arranged on opposite sides ofsaid first linear rail and a second set of rollers arranged on oppositesides of said second rail.
 13. The system as recited in claim 12,further comprising: a first actuator which, when activated, exerts aforce urging said mobile platform to move; an extendible arm comprisinga first member pivotably mounted to said frame of said mobile platformand a second member which is translatable relative to said first member;a second actuator which, when activated, exerts a force urging saidextendible arm to pivot relative to said frame; an NDI sensor unitcoupled to a distal end of said second member; and an encoder devicecapable of outputting signals representing incremental movements of saidNDI sensor unit along a substructure.
 14. The system as recited in claim13, further comprising a third actuator which, when activated, exerts aforce urging said second member to translate relative to said firstmember.
 15. The system as recited in claim 13, further comprising acompliant support structure between said NDI sensor unit and said distalend of said second member.
 16. The system as recited in claim 13,wherein said NDI sensor unit is an ultrasonic transducer array.
 17. Thesystem as recited in claim 13, further comprising a pivot joint by whichsaid extendible arm is pivotably mounted to said frame of said mobileplatform, wherein said first and second extendible supports and saidbridge beam are configured such that said bridge beam, said first andsecond cross beams, and said first and second linear tracks aretranslatable and rotatable between a first location in which said mobileplatform is upright and said pivot joint has an elevation and a secondlocation in which said mobile platform is upside down and said pivotjoint has the same elevation.
 18. A method for scanning a stringer in aninside of a workpiece having a curved section that extendslongitudinally and circumferentially using the system as recited inclaim 13, the method comprising: placing the curved cylinder-likeworkpiece so that the workpiece overlies the bridge; moving the mobileplatform to a starting position along the bridge and underneath theworkpiece; pivoting the extendible arm to a target tilt angle dependenton an angular position of a stringer to be inspected; at the target tiltangle, extending the second member of the extendible arm until the NDIsensor unit engages a first portion of the stringer to be inspected;activating the NDI sensor unit to inspect the first portion of thestringer; processing signals output from the NDI sensor unit to derivefirst scan data characterizing a structural state of the first portionof the stringer; and acquiring first location data representing a firstlocation of the NDI sensor unit when the NDI sensor unit engages thefirst portion of the stringer.
 19. The method as recited in claim 18,further comprising: moving the mobile platform in a manner that causesthe NDI sensor unit to travel along the stringer until the NDI sensorunit engages a second portion of the stringer; activating the NDI sensorunit to inspect the second portion of the stringer; processing signalsoutput from the NDI sensor unit to derive second scan datacharacterizing a structural state of the second portion of the stringer;and acquiring second location data representing a second location of theNDI sensor unit when the NDI sensor unit engages the second portion ofthe stringer.
 20. The method as recited in claim 18, wherein the firstscan data is acquired from at least a central cap portion of thestringer and an angled side of the stringer.